Device for use of lithium haloboracites as solid electrolytes

ABSTRACT

Lithium haloboracites of the formula Li 4  B 7  O 12  X where X is Cl, Br, I or a mixture thereof are useful as solid electrolytes for galvanic cells. Preferred lithium haloboracites are those in which X is a mixture of Br and Cl where Br is 20 to 50 mole percent of X.

CROSS-REFERENCE TO RELATED APPLICATION

This is a division of application Ser. No. 408,278 filed Oct. 23, 1973, now U.S. Pat. No. 3,911,085.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to lithium haloboracites and to their use as solid electrolytes in galvanic cells.

2. Description of the Prior Art

Li has long been regarded as a desirable component of galvanic cells. Its unparalleled reduction potential (E° _(red) =-3.024v) and its light weight have often suggested its use as the anodic component in high energy-density batteries. Its reactivity with water has required the use of non-aqueous electrolytes such as organic-soluble lithium perchlorates, or fused lithium halides usable only at high temperature.

Solid electrolytes and in particular completely solid state galvanic cells offer special possibility for long shelf life, broad temperature limits of operability and miniaturization. The requirement for good ionic transport and very low electronic conductivity has led primarily to the use of various silver halides as solid electrolytes. The relatively low electrochemical potential of Ag/Ag+ and the relatively high density and cost are disadvantages. Alternatives to the solid state transport of Ag+ are the more recent discovery of transport by alkali metal cations in beta-aluminas and anion transport in calcia-stabilized zirconia at very high temperatures.

Two lithium haloboracites of the formula Li₄ B₇ O₁₂ Cl and Li₄ B₇ O₁₂ Br have been previously reported by Lavasseur et al. in Mater. Res. Bull. 6, 15-22 (1971); and by Jeitschko et al. in Z. Naturforsch, 27b, 1423 (Nov. 1972). They were described as related to the well-known boracites of divalent metals typified by the mineral boracite, Mg₃ B₇ O₁₃ Cl. No mention is made of mixed halogen phases or electrical properties.

SUMMARY OF THE INVENTION

This invention relates to the discovery that lithium haloboracites of the formula Li₄ B₇ O₁₂ X wherein X is Cl, Br, I or mixtures thereof are useful as solid electrolytes. More particularly, this invention provides a device for transporting lithium ions which comprises (a) means for supplying lithium ions, (b) means for removing lithium ions, said means for supplying and removing lithium ions being separated by (c) a solid electrolyte consisting essentially of a lithium haloboracite Li₄ B₇ O₁₂ X where X is Cl, Br, I or a mixture thereof, and (d) electrode means to complete an electrical circuit between said supply and said removal means.

This invention also relates to novel lithium haloboracites of the formula Li₄ B₇ O₁₂ X wherein X is I_(1-m) Br_(m) where m is 0 to 0.5 or X is Cl_(1-n) Br_(n) where n is 0.2 to 0.5.

DETAILED DESCRIPTION OF THE INVENTION

The lithium haloboracites used in accordance with this invention are crystalline solids in which unusual ionic conductivity has been found to occur. We have discovered that the lithium ions in these haloboracites, unlike the divalent metal ions in ordinary haloboracites, are quite mobile and permit substantial ionic conductivity, especially at slightly elevated temperatures. Moreover, the novel haloboracites of the formula Li₄ B₇ O₁₂ Cl_(1-n) Br_(n) where n is 0.2 to 0.05 exhibit unusually high Li+ mobility at unexpectedly low temperatures.

The use of lithium haloboracites as solid electrolytes makes possible for example a primary galvanic cell to provide a lightweight source of stable voltage for low current drain at ambient temperatures. In other applications the lithium haloboracite solid electrolytes may be used at higher temperatures with correspondingly higher conductivity in a lightweight power source employing for example liquid lithium anodes. In still other applications the lithium haloboracites may be used as a diaphragm in the electrolytic preparation of lithium metal.

Preparation of Li₄ B₇ O₁₂ X

The known compositions Li₄ B₇ O₁₂ Cl and Li₄ B₇ O₁₂ Br can be obtained in the form of microcrystalline powders by heating at about 650°C mixtures of B₂ O₃ /Li₂ O in molar ratios from about 7/4 to about 7/2 in an excess of the fused salts, LiCl or LiBr. The method does not appear to work with LiI. The haloboracite compounds can be separated from other components of the reactant mixture by washing with water in which the lithium haloboracites are relatively insoluble. Their identity and purity can be determined by analysis and from the characteristic x-ray diffraction patterns.

To obtain the lithium haloboracites as clear crystals of macroscopic size it is preferred to use hydrothermal methods to promote crystal growth. For the chloro- and bromo-boracites a pressure of about 3 kbar suffices (1 kbar is 10⁹ dynes/cm² or about 1000 atmospheres), while for iodo-boracites higher pressures of about 30 kbar are used. After contact of the reactants for several hours at about 700°C. under pressure the mixture should be slowly cooled (e.g., 10°-30°C. per hour) for at least about 8 hours in order to prolong conditions for crystal growth.

Reactions at 3 kbars are conveniently carried out by sealing the appropriate reactants into gold tubes. These tubes are then placed in a bomb, pressurized with argon, and heated. The argon pressure and bomb temperature are adjusted to give the desired reaction conditions. Reactions under higher pressure may be carried out in a piston-cylinder device like that described by Boyd and England in J. Geophys. Res., 65,741-8 (1960) where the reactants are sealed in a container made of platinum or gold tubing.

Boric oxide and boric acid are acceptable sources of boron, while lithium oxide or hydroxide can provide a source of lithium in addition to the appropriate lithium halides. Alternatively, lithium borates may be used as a source of both lithium and boron. The atomic ratio Li/B in the starting mixture may depart somewhat from the theoretical 0.57, for example it may be from about 0.33 to about 0.66. Similarly the ratio X/B (theory 0.14) may range from about 0.12 to about 0.5. A small amount of water for example sufficient to provide a H₂ O/B ratio of about 0.2-0.5, is useful to provide good hydrothermal growth conditions. Water is conveniently added as a hydrate of one of the reactants. It will be appreciated that small changes in acidity may vary solublity relations in the aqueous growth media.

When lithium boracites with mixed halogens are prepared the proportion of the various halogens in the product is determined principally by their respective proportions in the starting mixture. With the mixed halogens Cl and Br there is a tendency for a larger proportion of the small halogen, Cl, to be incorporated in the product. The proportion of two halides in the product may be determined by analysis. The proportion may also be inferred in accordance with Vegard's law from the unit cell dimension relative to those of the end members, i.e., the pure lithium haloboracites with a single halogen which form the solid solution containing mixed halides.

Structure of Li₄ B₇ O₁₂ X

The identity of the lithium haloboracites can be determined from characteristic x-ray diffraction patterns. The Hagg-Guinier powder patterns may be indexed on the basis of a cubic lattice with unit cell dimension a = 12.144A for Li₄ B₇ O₁₂ Cl and a = 12.190A for Li₄ B₇ O₁₂ Br as described in the Levasseur et al. reference previously cited. We have found that the new analogue Li₄ B₇ O₁₂ I has a similar cubic pattern with a = 12.273A and that the mixed Cl-Br and Br-I compositions form solid solutions with unit cell dimensions between those of the respective end-members of each series.

Despite the apparently successful assignment of cubic structure, optical examination of lithium haloboracite crystals under polarized light shows them to be anistropic at room temperature, that is, their optical properties vary depending on the direction in which they are observed. Critical examination of the Hagg-Guinier powder patterns of these compounds shows some resolved line splitting in the x-ray films for Li₄ B₇ O₁₂ Br and from the splitting character and intensity distribution, rhombohedral symmetry is indicated. A least-square refinement of the resolved lines gives the rhombohedral cell dimensions = 12.187A, α = 90.07A. Thus, at room temperature the structure of each has a symmetry lower than cubic indicating that they exist in a slightly distorted version of a higher temperature prototype cubic structure which is optically isotropic and piezoelectric.

The transition between the distorted pseudocubic phase at low temperature and the high temperature cubic phase may be observed optically using polarized light on a hot stage microscope. The temperature of transition may also be found by differential scanning calorimetry. Structural distinctions between the high- and low-temperature forms and their consequences are illustrated below for the case of Li₄ B₇ O₁₂ Cl.

The transition temperature of solid solutions of the haloboracites is generally between those of its component haloboracites. However, substitution of bromine for chlorine in Li₄ B₇ O₁₂ Cl first causes a surprising lowering of the transition before the expected trend toward the much higher transition of Li₄ B₇ O₁₂ Br takes over after about one third substitution by bromine. While Li₄ B₇ O₁₂ Cl is normally in its low temperature, optically anisotropic form at room temperature, its transition from the high temperature cubic form may be sluggish upon cooling. By means of the slight substitution of bromine for chlorine the isotropic form may be further stabilized at room temperature. The unexpected variation of transition temperature with composition is shown in FIG. 1, where the mole fraction of bromine is determined from the regular variation in lattice constant of the cubic or pseudo-cubic structures. It is clear from this figure that the benefits of the more conductive, cubic structure can be extended to a most useful temperature range by substituting bromine for chlorine to the extent of 20-50 atom percent.

X-ray studies of these lithium haloboracites carried out at room temperature and elevated temperatures show that lithium occupies two crystallographic sites which are close to each other. This makes available channels for the migration of Li ions in all three dimensions. The occupancy of the two sites changes with temperature thus indicating that migration indeed does happen from one site to the other.

Electrical Properties of Li₄ B₇ O₁₂ X

The indications of ionic mobility provided by crystallographic studies are borne out by electrical measurements. Single crystals of the lithium haloboracites were electroded using either a commercial silver paste or gold sputtering. Conductance and capacitance of the crystals were measured using an automatic capacitance bridge (Hewlett Packard model 4270A) at frequencies ranging from 10³ to 10⁶ Hertz. Data were usually taken continuously at increasing temperature after first cooling to liquid nitrogen temperature. The Li₄ B₇ O₁₂ X crystals all showed unusually high a.c. conductivity particularly at temperatures above the pseudo-cubic to cubic transition. A marked increase in the a.c. conductivity occurs at the temperature of transition.

Typical results of conductivity (measured at a frequency of 10⁵ Hertz) versus the reciprocal of absolute temperature (1000/T) are shown in FIG. 2 where curve A represents data for Li₄ B₇ O₁₂ Br, curve B for Li₄ B₇ O₁₂ Cl, and curve C for Li₄ R₇ O₁₂ Cl₀.68 Br₀.32. The dashed curve represents data for lithium iodide, a well-known ionic conductor used commercially. All of the lithium haloboracites have useful conductivities, but those compositions Li₄ B₇ O₁₂ Cl_(1-n) Br_(n) where n has values between 0.20 and 0.50 are preferred because of unusually high conductivity particularly at room temperature. The effect of frequency on a.c. conductivity is moderate below the transition temperature, increasing by a factor of about 25x as frequency increases from 10³ to 10⁶ Hz. Above the transition temperature, frequency is even less influential causing only about 5x increase in conductivity over the same range.

The ionic character of the conduction process is further indicated by the change of capacitance with frequency. Well below the transition temperature the measured capacitance is frequency independent, but above the transition temperature it is inversely proportional to the frequency, decreasing by 3 orders of magnitude over the range 10³ -10⁶ Hz. FIG. 3 shows capacitance vs. temperature for crystals of Li₄ B₇ O₁₂ Cl₀.68 Br₀.32 (curves D) and Li₄ B₇ O₁₂ Br (curves E) at various frequencies, f.

The ionic conductivity may be directly demonstrated by the use of lithium haloboracite crystals to separate anodic and cathodic compartments in galvanic devices. Examples 20 and 21 illustrate their use in low power, primary voltage sources particularly adapted for convenient use at room temperature. The slope of the lines in FIG. 2, indicating an activation energy of about 0.6 electron volts, suggests utility as a power source at elevated temperatures, particularly above 300°C. where the ionic conductivity approaches that of the best known solid electrolytes.

It is also contemplated that the lithium haloboracites may be used in electrolytic devices wherein for example they may be arranged as a membrane across which a potential may be applied in order to selectively transport lithium. Thus, for example, lithium haloboracites may be used as separators to purify lithium from mixtures with other alkali metals or their ions.

EXAMPLES

The following examples, illustrating the preparation of lithium haloboracites and in particular the novel lithium haloboracites of this invention and their use as solid electrolytes are given without any intention that the invention be limited thereto.

EXAMPLE 1

A mixture of 0.551 g. LiCl (13 mmoles), 0.932 g. LiOH.H₂ O (22 mmols), and 3.627 g. B₂ O₃ (52 mmols) was sealed into a 1/2 inch I.D. gold tube about 5 inches in length. This tube was maintained for 4 hours at a temperature of 700°C under an external argon pressure of approximately 3 kbars, was then slowly cooled at a rate of approximately 25°C/Hr. The resultant solids were extracted with water to remove soluble impurities and clear, colorless, polyhedral crystals of variable size up to several mm. across were isolated. X-ray diffraction powder data obtained with a Hagg-Guinier camera upon these crystals indicated the pattern of the boracite-type compound Li₄ B₇ O₁₂ Cl which could be indexed on the basis of a cubic cell of unit dimension a = 12.144A.

Optical examination of these crystals under polarized light showed that they were anisotropic at room temperature but became isotropic upon gentle warming. By differential scanning calorimetry, two endotherms peaking around 37° and 75°C. were observed on upheat. Upon cool down, the upper transition was rapidly reversible, but the lower transition was sluggish. The change from optical anisotropy to isotropy accompanied the lower of the two transitions. By differential thermal analysis, the onset of decomposition of Li₄ B₇ O₁₂ Cl was observed at a temperature around 870°C.

EXAMPLE 2

A mixture of 0.721 g. LiCl (17 mmols), 1.458 g. LiBo₂.2H₂ O (17 mmols), 1.785 g. B₂ O₃ (25 mmols), and 1.054 g. H₃ BO₃ (17 mmols) was sealed into a 1/2 inch I.D. gold tube about 5 inches in length and reacted as in Example 1. The resultant solids were extracted with water to remove soluble impurites and a mixture of crystals, some clear and some having a crazed appearance and having growth habits varying from pseudo-cubes to polyhedra to platelets up to several mm. across, was isolated. X-ray diffraction powder data indicated this mixture of crystal habits to have the pattern of the boracite-type compound, Li₄ B₇ O₁₂ Cl,described in Example 1. Analysis on these crystals confirmed the composition Li₄ B₇ O₁₂ Cl.

Calculated: Li = 8.39; B = 22.87; O = 58.03; Cl = 10.71 Found: Li = 8.05; B = 23.40; O = 58.62; Cl = 10.57

The crystals were optically anisotropic, but became isotropic after gently warming on a hot plate. After standing without heat for 2 hours the crystals slowly began to exhibit anisotropic behavior.

EXAMPLE 3

A mixture of 0.848 g. LiCl (20 mmols), 0.858 g. LiBO₂.2H₂ O (10 mmols), 1.050 g. B₂ O₃ (15 mmols), and 0.620 g. H₃ BO₃ (10mmols) was sealed into a 3/8 inch I.D. gold tube about 5 inches in length and reacted in a manner similar to that described in Example 2. The resultant solids were extracted with water to remove soluble impurities. A mixture of crystals was obtained similar in appearance to that obtained in Example 2 and having the X-ray diffraction powder pattern of Li₄ B₇ O₁₂ Cl as observed in Examples 1 and 2. Analysis indicated 8.05% Li and 58.62% O.

An infrared spectrum showed the presence of trigonally- and tetrahedrally-coordinated boron, both of which are present in the boron-oxygen network of the boracite structure. No bands indicating the presence of OH or H₂ O were observed. These crystals of Li₄ B₇ O₁₂ Cl were observed. These crystals of Li₄ B₇ O₁₂ Cl were observed to give a strong positive test for piezoelectricity when examined with a transmission-type piezoelectricity detector. The a.c. conductivity of a crystal is shown in FIG. 2, curve B.

EXAMPLE 4

A mixture of 1.300 g. LiBr (15 mmoles), 1.260 g. LiOH.H₂ O (30 mmoles), and 4.200 g. B₂ O₃ (60 mmoles) was reacted in a sealed gold tube at 700°C and 3 kbars pressure using the procedure of Example 1. The resultant solids were extracted with water to remove soluble impurities and crystals up to several mm. across having clear to crazed appearances and generally a cubic-like growth habit were isolated with some irregularly shaped, clear shards which proved to be Li₂ B₄ O₇. X-ray diffraction powder data obtained with a Hagg-Guinier camera upon crystals of cubic-like growth habit indicated the pattern of the boracite-type compound Li₄ B₇ O₁₂ Br which could be indexed on the basis of a cubic cell of unit dimension a = 12.190A. Critical examination of these x-ray films showed some resolved line splitting and from the splitting character and intensity distribution, rhombohedral symmetry was indicated. A least-squares refinement of the resolved lines gave a = 12.187A, γ = 90.07°.

Optical examination confirmed the anistropy of Li₄ B₇ O₁₂ Br at room temperature and differential scanning calorimetry on crystals of cubic-like habit showed the presence of a single reversible transition that exhibited a sharp endotherm on upheat peaking around 227°-232°C. This Li₄ B₇ O₁₂ Br boracite differs from Li₄ B₇ O₁₂ Cl in that only one rather than two transitions are obtained from heating from room temperature to the high-temperature cubic form. Crystals of Li₄ B₇ O₁₂ Br gave a strong positive test for piezoelectricity when examined with a transmission-type piezoelectricity detector. Conductivity and dielectric properties of a crystal are shown in FIG. 2, curve A, and FIG. 3, curve B, respectively.

EXAMPLE 5

A mixture of 1.285 g. LiBr (14.8 mmoles), 1.269 g LiBO₂.2H₂ O (14.8 mmoles), 1.554 g. B₂ O₃ (22.2 mmoles), and 0.918 g. H₃ BO₃ (14.8 mmoles) was reacted using the procedure of Example 4 except that the product was slowly cooled to a temperature of about 100°C. The resulting Li₄ B₇ O₁₂ Br was isolated as described in Example 4.

EXAMPLE 6

A mixture of 1.181 LiBr (13.6 mmoles), 0.583 g. LiBO₂.2H₂ O (6.8 mmoles), 0.714 g. B₂ O₃ (10.2 mmoles), and 0.422 g. H₃ BO₃ (6.8 mmoles) was reacted using the procedure of Example 4 except that the product was slowly cooled to a temperature of about 100°C. The resulting Li₄ B₇ O₁₂ Br was isolated as described in Example 4.

EXAMPLE 7

A 0.806 g. pellet made from a mixture of 1.606 g. LiI (12 mmols), 0.515 g. LiBo₂.2H₂ O (6 mmols), 0.630 g. B₂ O₃ (9 mmols), and 0.372 g. H₃ BO₃ (6 mmols) and contained in a welded Pt capsule was pressured to 30 kbars in a piston-cylinder device, heated for 4 hours at about 700°C, slow cooled for 11 hours to about 560°C, and quenched to room temperature. Following extraction of this reaction product with water and then acetone, hazy, colorless crystals of cubic-like habit were isolated. X-ray diffraction powder data obtained with a Hagg-Guinier camera upon these crystals showed a boracite-type pattern (Table I) isotypic with that for Li₄ B₇ O₁₂ Cl, thus indicating formation of the new compound Li₄ B₇ O₁₂ I. These x-ray data could be indexed on the basis of a cubic cell of unit dimension a = 12.273A.

Optical examination of these crystals showed them to be anisotropic at room temperature, and differential scanning calorimetry showed the presence of a single reversible transition that exhibited a sharp endotherm on upheat peaking around 270°C. Crystals of Li₄ B₇ O₁₂ I gave a strong positive test for piezoelectricity when examined with a transmission type piezoelectricity detector.

                  TABLE I                                                          ______________________________________                                         X-Ray Diffraction Powder Pattern of Li.sub.4 B.sub.7 O.sub.12 I                ______________________________________                                         Intensity*     h     k     l     d Spacing, A                                  ______________________________________                                         50             2     0     0     6.1377                                        70             2     2     0     4.3395                                        100            2     2     2     3.5437                                        60             4     0     0     3.0683                                        100            4     2     0     2.7447                                        95             4     2     2     2.5053                                        50             4     4     0     2.1698                                        70             5     3     1     2.0747                                        60             6     0     0     2.0458                                        60             6     2     0     1.9407                                        40             6     2     2     1.8505                                        30             4     4     4     1.7717                                        50             6     4     0     1.7020                                        70             6     4     2     1.6400                                        10             7     3     1     1.5976                                        20             8     0     0     1.5341                                        70             8     2     0     1.4884                                        30             6     6     0     1.4463                                        10             7     5     1     1.4165                                        20             8     4     0     1.3720                                        30             8     4     2     1.3389                                         5             6     6     4     1.3080                                        ______________________________________                                          *An intensity value of 100 is assigned to the strongest lines of the           pattern.                                                                 

EXAMPLE 8

A mixture of 0.276 g. LiCl (6.51 mmol), 0.566 g. LiBr (6.52 mmol), 1.110 g. LiOH.H₂ O (26.46 mmol), and 3.627 g. B₂ O₃ (51.81 mmol) was sealed into a 1/2 inch I.D. gold tube about 5 inches long and was reacted in the manner of Example 1. Following extraction of the resultant solids with water, opaque to clear polyhedral crystals were isolated. A Debye-Scherrer x-ray diffraction powder pattern obtained on these crystals showed them to be isotypic with the boracite-type compounds Li₄ B₇ O₁₂ Cl and Li₄ B₇ O₁₂ Br of Examples 1-7. This pattern (Table II) could be indexed on the basis of a face-centered cubic cell of unit dimension a = 12.159A and indicates formation of a mixed chloro-bromo lithium boracite, Li₄ B₇ O₁₂ (Cl, Br), intermediate between the two end members of cell size a_(Cl) = 12.144A and a_(Br) = 12.190A. Based on a linear variation of cell constant with composition according to Vegard's law the composition is Li₄ B₇ O₁₂ Cl₀.68 Br₀.32.

In contrast (1) to Li₄ B₇ O₁₂ Cl which is anisotropic at room temperature and shows two transitions upon heating, and (2) to Li₄ B₇ O₁₂ Br which is also anisotropic at room temperature and shows one transition upon heating, crystals of this mixed chloro-bromo lithium boracite were essentially all isotropic at room temperature and showed no transition by differential scanning calorimetry when examined over the temperature range -110° to +550°C.

                  TABLE II                                                         ______________________________________                                         X-Ray Diffraction Powder Pattern of The Mixed Halogen                          Li.sub.4 B.sub.7 O.sub.12 (Cl,Br) Prepared in Example 8                        ______________________________________                                         Intensity*     h     k     l     d Spacing, A                                  ______________________________________                                         25             2     0     0     6.0978                                        95             2     2     2     3.5143                                        30             4     0     0     3.0416                                        95             4     2     0     2.7205                                        80             4     2     2     2.4834                                        50             4     4     0     2.1495                                        100            5     3     1     2.0558                                        10             6     0     0     2.0276                                        30             6     2     0     1.9225                                        10             6     2     2     1.8331                                        30             4     4     4     1.7549                                        40             6     4     0     1.6861                                        50             6     4     2     1.6248                                        35             7     3     1     1.5828                                        10             8     0     0     1.5192                                        50             8     2     0     1.4743                                        20             7     5     1     1.4042                                        20             6     6     2     1.3944                                        10             8     4     0     1.3590                                        10             9     1     1     1.3357                                        10             8     4     2     1.3266                                        10             6     6     4     1.2953                                        15             9     3     1     1.2742                                        40             8     4     4     1.2409                                         5             10    0     0     1.2153                                        25             10    2     0     1.1922                                        30             10    2     2     1.1699                                        10             10    4     0     1.1291                                         2             8     8     0     1.0749                                        10             11    3     1     1.0623                                        10             8     8     2     1.0578                                        15             10    6     0     1.0422                                        10             11    3     3     1.0306                                        20             10    6     2     1.0274                                        20             12    0     0     1.0130                                        10             11    5     1     1.0034                                        ______________________________________                                          *An intensity value of 100 is assigned to the strongest line of the            pattern.                                                                 

EXAMPLE 9

A mixture of 0.361 g. LiCl (8.51 mmol), 0.739 g. LiBr (0.851 mmol), 1.458 g. LiBO₂.2H₂ O (17.0 mmol), 1.785 g. B₂ O₃ (25.5 mmol), and 1.054 g. H₃ BO₃ (17.0 mmol) was sealed into a 1/2 inch I.D. gold tube about 5 inches long and was reacted in the manner of Example 2. Crystals of cube-like growth habit were isolated. X-ray diffraction powder data were essentially the same as those in Example 8, indicating formation of a mixed chloro-bromo lithium boracite, Li₄ B₇ O₁₂ (Cl, Br). A Guinier x-ray pattern could be indexed on the basis of a face-centered cubic cell of unit dimension a = 12.157A indicating by Vegard's law a Cl/Br atom ratio of 72/28.

Optical examination of these crystals showed them to be anisotropic at room temperature but isotropic after slight warming. Differential scanning calorimetry showed upon very gentle heating the presence of a single transition in this Li₄ B₇ O₁₂ (Cl,Br) boracite that peaked at about 31°C. Crystals of this composition gave a strong positive test for piezoelectricity when examined with a transmission-type piezoelectricity detector.

EXAMPLES 10-19

Lithium haloboracites having various proportions of Cl/Br were prepared in the same manner as described for Example 2 by mixing in each case 1.458 g. LiBO₂.2H₂ O, 1.785 g. B₂ O₃, and 1.054 g. H₃ BO₃ with the amount of LiCl and LiBr shown in Table III. Following extraction of the product with water, mixtures of crystals of varying growth habits and degrees of clarity were obtained. Clear crystals having an irregular shard-like habit generally proved to be Li₂ B₄ O₇ whereas those of cubic-like or polyhedral habit were observed by x-ray diffraction powder data to have patterns isotypic with the boracites Li₄ B₇ O₁₂ Cl and Li₄ B₇ O₁₂ Br and from their interplanar spacings to have unit cells intermediate in size between those of the two end members, indicating formation of the mixed chloro-bromo lithium boracites Li₄ B₇ O₁₂ (Cl,Br). Differential scanning calorimetry demonstrated the presence of a transition in these boracites. The temperature at which the maximum endotherm appears on upheat is indicated in the final column of Table III. These materials were all anisotropic below the indicated transition temperature. The conductivity and dielectric properties of a crystal from Example 12 are shown in FIG. 2, curve C and FIG. 3, curve D, respectively.

                                      TABLE III                                    __________________________________________________________________________     Li.sub.4 B.sub.7 O.sub.12 Cl.sub.1.sub.-n Br.sub.n Boracites                   __________________________________________________________________________     Reactants               Product                                                                        Cubic Cell     Transition                              Example          Atom Ratio                                                                            Edge   Fraction Br                                                                            Temp                                    No   gLiCl gLiBr Cl/Br  a.     n**     °C                               __________________________________________________________________________     10   0.649 0.148 90/10  12.150 0.13    42                                      11   0.541 0.369 75/25  --     --      46                                      12   0.361 0.739 50/50  12.159 0.32    16                                      13   0.180 1.107 25/75  --     --      47                                      14   0.144 1.181 20/80   12.163*                                                                              0.41    58                                      15   0.108 1.255 15/85   12.166*                                                                              0.48    91                                      16   0.072 1.329 10/90   12.166*                                                                              0.48    102                                     17   0.072 1.329 10/90   12.168*                                                                              0.52    116                                     18   0.036 1.403  5/95   12.176*                                                                              0.69    160                                     19   0.018 1.440 2.5/97.5                                                                              --     --      197                                     __________________________________________________________________________      *values from cube root of rhombohedral cell volume                             **from Vegard's law                                                      

EXAMPLE 20

The ionic transport of lithium through a lithium haloboracite was shown by the use of an electrochemical concentration cell. This cell was composed of (1) a saturated lithium amalgam in one leg and (2) a 0.1 saturated lithium amalgam (1 part saturated amalgam, 9 parts mercury) in the other leg. The two amalgams were separated by a crystal platelet of Li₄ B₇ O₁₂ Cl₀.68 Br₀.32 of Example 12. This cell produced an open circuit voltage of 68 mV measured by a high impedance voltmeter between electrodes in the two legs and showed a direct current conductance of about 10⁻ ⁷ ohms⁻ ¹.

When the solid electrolyte crystal was replaced by a liquid ionic conductor comprising LiClO₄ dissolved in an organic solvent the open circuit voltage was scarcely different (70 mV) indicating ionic conduction in the lithium haloboracite. Opposing surfaces of the crystal were then sputtered with gold and the direct current conductance now measured less than 10⁻ ¹⁰ ohms⁻ ¹ indicating that the electronic component of the d.c. conductivity was no more than a thousandth of the ionic component. Thus the lithium ionic transport number is greater than 0.99 and the lithium haloboracites are useful as battery components.

EXAMPLE 21

A primary battery comprising a lithium haloboracite solid electrolyte was constructed essentially as shown in FIG. 4. In this drawing 1 is a lithium haloboracite material; 2 is a lithium foil anode wrapped around a platinum wire 4; 3 is a cuprous sulfide cathodic layer compacted around a platinum wire; 5 is a commercial potting resin; and 6 is a mounting block of polymethylmethacrylate.

In this particular example the solid electrolyte was a crystal of Li₄ B₇ O₁₂ Cl₀.68 Br₀.32 as described in Example 12. After assembly of the two electrodes and mounting blocks around the crystal the "sandwich" was compressed to insure intimate contact, and while compressed, was hermetically sealed by the potting resin with only the platinum wire terminations exposed. The assembly produced an open cell voltage of 3.0 volts when measured by a high impedance voltmeter, and showed good shelf stability.

This configuration is a useful primary cell wherein a lithium haloboracite transports lithium ions between an electrode source of lithium and a cuprous sulfide electrode capable of reacting with lithium.

Although the invention has been described and exemplified by way of specific embodiments, it is not intended that it be limited thereto. As will be apparent to those skilled in the art, numerous modifications and variations of these embodiments can be made without departing from the spirit of the invention or the scope of the following claims. 

We claim:
 1. An electrochemical device which comprisesa. a lithium-containing anode which is capable of supplying lithium ions, b. a cathode which is capable of removing lithium ions, said anode and cathode being separated by c. a solid electrolyte consisting essentially of a lithium haloboracite Li₄ B₇ O₁₂ X where X is Cl, Br, I or a mixture thereof.
 2. The device of claim 1 wherein it is a galvanic cell.
 3. The device of claim 1 wherein the lithium haloboracite is of the formula Li₄ B₇ O₁₂ Cl_(1-n) Br_(n) where n is 0.2 to 0.5.
 4. The device of claim 3 wherein it is a galvanic cell.
 5. The galvanic cell of claim 2 wherein the anode is liquid lithium.
 6. The galvanic cell of claim 2 wherein the anode is a lithium amalgam.
 7. The galvanic cell of claim 6 wherein the anode is a saturated lithium amalgam and the cathode is a 0.1 saturated lithium amalgam.
 8. The galvanic cell of claim 2 wherein the anode is lithium foil.
 9. The galvanic cell of claim 8 wherein the cathode is cuprous sulfide.
 10. The device of claim 1 wherein it is an electrolytic cell for purifying lithium in which the solid electrolyte is in the form of a separator between the anode and the cathode. 